In known conventional laser fusion cutting operations, a laser beam produced with a CO2 laser or a solid-state laser melts the material and inert gas (for example, nitrogen or argon) is used to blow the molten mass and cinders downwards out of the cutting joint at high pressure. The characteristics of the melt bath and the melt flow vectors that occur therein are dependent on absorption. In contrast to conventional laser fusion cutting operations, with so-called “keyhole” laser fusion cutting not only is the material melted, but vapor is also produced. When the vapor is discharged, it applies pressure to the molten mass and ousts it, whereby a narrow vapor capillary (keyhole) is produced in the material. The vapor capillary is surrounded by the molten mass and moves with the laser beam through the workpiece.
When cutting steels using CO2 lasers, it may be assumed that, in view of the narrow Brewster angle maximum, only a locally limited region of the cutting front absorbs the laser radiation in an optimum manner. Thus, the surface temperature of the molten mass does not reach the boiling point. In the case of relatively thick metal sheets, the molten mass is thereby expelled primarily at the apex of the cutting front. This results in a melt flow vector that changes only slightly along the cutting front, because there is no surface vaporization in this instance. The result is a very orientated, compact melt discharge cone, as is known in CO2 fusion cutting.
When a solid-state laser is used with a wavelength in the region of approximately 1 μm, the Brewster angle and the position of the Brewster maximum, and consequently the absorption, change in comparison with the CO2 laser with a wavelength of 10.6 μm. The changed absorption along the cutting front leads locally to surface evaporation with high temperature gradients. This results in changes of the temperature-dependent surface tension of the molten mass at the apex of the cutting front and consequently also in melt flow vectors that have not only exclusively vertical, but also horizontal portions. These flow vectors become less stable over time and lead to a poor base corrugation appearance of the cutting edge and to significant cutting edge roughness known in solid-state laser fusion cutting operations.
Previous laser beam fusion cutting operations using solid-state lasers to cut high-grade steels having a sheet thickness of more than 2 mm generally have a poor cutting edge quality, with significant burr formation and oxidation at the cutting flanks. Possible process windows are often very small in comparison with CO2 laser beam processes and furthermore, in particular in the central and thick sheet regions, poor roughness values result at the cutting flank.